Groot is a Flora colossus from Planet X the capital of the branch worlds. The sapling that will come to be known as "Groot" came from an "Enobled Sap-line" and gifted in quasi-dimensional super-positional engineering.
Groot did not get along with his fellow saplings but instead preferred the company of the "Maintenance Mammals" who the other saplings treated with prejudice. After Groot killed another sapling to defend a maintenance mammal it was brutalising, he was exiled of the planet by the "Arbor Masters".
Into his adulthood Groot found himself in Kree space, where he was promptly captured and imprisoned. While in jail he formed a rapport with Rocket Raccoon and was assigned to a covert ops team lead by Starlord, whose goal was to assault Hala and defeat the Phalanx. His body was mostly destroyed, but a small twig of him split off, preserving him, traveling in the care of Rocket Raccoon, fitting easily in his hand. He can grow back to his full size in time.